Add production features: slog adapter, scan helpers, slow query logging, pool stats, tracer passthrough, test tx isolation

- slog.go: SlogLogger adapts *slog.Logger to dbx.Logger interface
- scan.go: Collect[T] and CollectOne[T] generic helpers using pgx.RowToStructByName
- cluster.go: slow query logging via Config.SlowQueryThreshold (Warn level in queryEnd)
- stats.go: PoolStats with Cluster.Stats() aggregating pool stats across all nodes
- config.go/node.go: NodeConfig.Tracer passthrough for pgx.QueryTracer (OpenTelemetry)
- options.go: WithSlowQueryThreshold and WithTracer functional options
- dbxtest/tx.go: RunInTx runs callback in always-rolled-back transaction for test isolation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dbxtest/tx_test.go

@@ -0,0 +1,50 @@
+package dbxtest_test
+
+import (
+	"context"
+	"testing"
+
+	"git.codelab.vc/pkg/dbx"
+	"git.codelab.vc/pkg/dbx/dbxtest"
+)
+
+func TestRunInTx(t *testing.T) {
+	c := dbxtest.NewTestCluster(t)
+	ctx := context.Background()
+
+	// Create a table that persists across the test.
+	_, err := c.Exec(ctx, `CREATE TABLE IF NOT EXISTS test_run_in_tx (id int)`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		_, _ = c.Exec(context.Background(), `DROP TABLE IF EXISTS test_run_in_tx`)
+	})
+
+	dbxtest.RunInTx(t, c, func(ctx context.Context, q dbx.Querier) {
+		_, err := q.Exec(ctx, `INSERT INTO test_run_in_tx (id) VALUES (1)`)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Row should be visible within the transaction.
+		var count int
+		err = q.QueryRow(ctx, `SELECT count(*) FROM test_run_in_tx`).Scan(&count)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if count != 1 {
+			t.Fatalf("expected 1 row in tx, got %d", count)
+		}
+	})
+
+	// After RunInTx returns, the transaction was rolled back; row should not exist.
+	var count int
+	err = c.QueryRow(ctx, `SELECT count(*) FROM test_run_in_tx`).Scan(&count)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if count != 0 {
+		t.Errorf("expected 0 rows after rollback, got %d", count)
+	}
+}